1. Field of the Invention
The present invention relates to an oil-impregnated sintered bearing obtained by impregnating a porous body formed of a sintered metal with a lubricant (lubricating oil or lubricating grease).
2. Description of the Related Art
In an oil-impregnated sintered bearing, a lubricating oil impregnated oozes out to the sliding portion (the portion of the bearing on which the shaft to be supported slides), and forms an oil film as a result of relative rotation between the bearing and the shaft. The shaft is supported by this oil film in a non-contact manner. As shown, for example, in FIG. 7, such an oil-impregnated sintered bearing is incorporated for use into a power transmission mechanism for a power window for opening and closing a window pane provided in a door panel of a vehicle (see, for example, JP2003-247548 A).
In the power transmission mechanism for a power window shown in FIG. 7, the torque of a motor 23 is reduced by a worm wheel mechanism 24, and the reduced torque is transmitted to a window pane opening/closing mechanism (not shown). In FIG. 7, a shaft 22 driven by the motor 23 is rotatably inserted into the inner periphery of one or a plurality of (three, in the example shown in the figure) oil-impregnated sintered bearings 21. The plurality of oil-impregnated sintered bearings 21 are fixed on predetermined coaxial positions in a casing 25. The worm wheel mechanism is composed of a worm gear portion 26 formed integrally with the shaft 22, and a wheel gear portion 27 which is in mesh with the worm gear portion 26 and is mechanically connected to a window pane opening/closing mechanism (not shown).
The torque input to the shaft 22 by the motor 23 is transmitted to the wheel gear portion 27 through the worm gear portion 26 of the shaft 22, and is further transmitted in a reduced state to the window pane opening/closing mechanism (not shown). As a result, the window pane, which is mechanically connected to the opening/closing mechanism, performs opening/closing motion in correspondence with the direction of the torque of the motor 23.
During operation of the above-mentioned power transmission mechanism for a power window, the lubricating oil which is impregnated in the oil-impregnated sintered bearings 21 oozes out to the sliding portions between the oil-impregnated sintered bearings 21 and the shaft 22, and oil films are formed by the lubricating oil thus oozing out. As a result, the shaft 22 is supported by the oil-impregnated sintered bearings 21 in a non-contact manner through the intermediation of the oil films, whereby a smooth sliding state is achieved between the components 21 and 22.
Generally speaking, in an oil-impregnated sintered bearing of this type, the inner diameter of the bearing surface provided in the inner periphery thereof is uniform over its entire axial dimension, and sliding (supporting) is effected between the shaft and the entire bearing surface.
However, in a certain type of power transmission mechanism, for example, in the power transmission mechanism shown in FIG. 7, the shaft 22 receives, due to the construction of the mechanism, a load from the wheel gear portion 27 through the worm gear portion 26, and is thereby deflected. Thus, as shown, for example, in FIG. 8, the outer peripheral surface (sliding surface) 22a of the shaft 22 may slide locally at an axial end of the bearing surface 21a of the oil-impregnated sintered bearing 21, and there is a fear of stress concentration being generated in the sliding region of the bearing surface 21a. Such stress concentration can lead to undesirable effects for the oil-impregnated sintered bearing, such as wear of the bearing surface of the oil-impregnated sintered bearing and generation of unusual noise.
JP 2003-120673 A discloses an oil-impregnated sintered bearing having tapered surfaces expanding its diameter in opposite directions at both axial ends of its inner periphery.
These tapered surfaces are formed as clearance portions for allowing relative oscillatory displacement (run-out motion) between the shaft and the oil-impregnated sintered bearing, and their inclination angle is normally set between 10 and 45 degrees.
Thus, in the oil-impregnated sintered bearing as disclosed in JP 2003-120673 A, when relative wobbling motion occurs between the shaft and the oil-impregnated sintered bearing, the outer peripheral surface of the shaft slides locally on the border portions between the tapered regions and the non-tapered region of the inner peripheral surface of the oil-impregnated sintered bearing, or slides locally on the tapered regions. As a result, there is the possibility of the stress concentration on the inner peripheral surface of the oil-impregnated sintered bearing not being mitigated to a sufficient degree.